Cook the spaghetti or rice noodles according to package instructions. Drain and set aside.

In a large skillet or wok, heat the vegetable oil over medium-high heat. Add the minced garlic and diced onion, sautéing until fragrant and the onion is translucent, about 2-3 minutes.

Add the ground beef to the skillet, breaking it up with a spatula. Cook until browned and fully cooked, about 5-7 minutes. Drain excess fat if necessary.

Stir in the sliced bell pepper and broccoli florets, cooking for another 3-4 minutes until the vegetables are tender but still crisp.

In a small bowl, mix together the soy sauce, oyster sauce, sesame oil, sugar, black pepper, and red pepper flakes (if using). Pour this sauce over the beef and vegetables, stirring to combine.

Add the cooked noodles to the skillet, tossing everything together until the noodles are well coated with the sauce and heated through, about 2-3 minutes.

Serve hot, garnished with sliced green onions.